Recently, many investigations have been made with regard to printing plates for a computer to plate system as the rapid technical progress in this field. In these investigations, as a lithographic printing plate precursor aiming at more rationalizing process and solving the waste solution treatment problem, a lithographic printing plate precursor capable of printing by being mounted on a printing machine as it is without being development processed after exposure has been investigated and various methods have been proposed.
As one of the methods of omitting processing steps, there is a method called on-machine development wherein an exposed printing plate precursor is mounted on a cylinder of the printing machine and while rotating the cylinder by supplying a fountain solution and an ink, a non-image area of the printing plate precursor is removed. That is, the method is a system in which after exposing a printing plate precursor, the printing plate precursor is mounted on a printing machine as it is and processing is finished in an ordinary printing step.
The lithographic printing plate precursor suitable for such an on-machine development is required to have a photosensitive layer soluble in a fountain solution and an ink solvent and also has a light-room handle ability suitable for being developed on a printing machine placed in a light room.
For example, in Japanese Patent No. 2938397, a lithographic printing plate precursor having formed on a hydrophilic support a photosensitive layer having fine particles of a thermoplastic hydrophobic polymer dispersed in a hydrophilic binder polymer is disclosed. In the patent, it is described that after exposing the lithographic printing plate precursor by an infrared laser to form images by coalescing the fine particles of the thermoplastic hydrophobic polymer by heat, the lithographic printing plate precursor having formed the images is mounted on a cylinder of a printing machine and can be subjected to an on-machine development by a fountain solution and/or ink.
Also, in JP-A-9-127683 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”) and WO 99/10186, it is described that after coalescing thermoplastic fine particles, a printing plate can be prepared by on-machine development.
However, in the method of forming images by welding, etc., of fine particles by heat as described above, there is a problem that the sensitivity is low or good press life is hard to obtain.